The existing image encryption algorithms encrypt images independently, which means the probability of an eavesdropper working out one image stays the same under the same circumstances (same bits of keys and the same algorithm, for example). However, in real-life applications such as communication systems, on one hand, sequences of images may need to be sent with various security demands, and on the other hand, the temporal order of sequences of images may be changed. This paper proposes a novel idea to integrate the notion of temporal order with cryptosystems. Experimental results show that satisfactory security performance is achieved when the order for the sequence of images is changed.